Chapter 4
Halogenation
 Addition of halogen to an alkane
o Only works on sp3-hybridized carbons
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Works best with chlorine and bromine
o Iodine is too slow
o Fluorine blows up (too fast)
Goes through radical intermediate, so putting halogen on more substituted carbon is
better
o Bromine is way more selective than chlorine
Initiation
o Homolytic cleavage of X2
2
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o One molecule splits into two radicals
Propagation
o Start with one molecule and one radical; end up with one molecule and one
radical!
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1)
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2)
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Termination
o Any two radicals stick together
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+ HCl
Reactive Intermediates
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Carbocations
o Carbon with three bonds and no lone pairs
o Trigonal planar geometry
o sp2-hybridized
o empty p-orbital
o stability - 3°>2°>1°> methyl
o electron-deficient
o only intermediate which can undergo rearrangement
hydride shift
1° carbocation with 3° position next door
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We’ll see a lot of this later in the course.
It’s a good idea to think “carbocations can rearrange” every time you see
a carbocation intermediate for the rest of your life!
Radicals
o Carbon with three bonds and one unpaired electron
o Trigonal planar geometry
o sp2-hybridized
o one electron in the p-orbital
o stability - 3°>2°>1°> methyl
o electron-deficient
Carbanions
o Carbon with three bonds and a lone pair
o sp3-hybridized
o Tetrahedral geometry
o Stability- methyl>1°>2°>3°
o Strong bases/nucleophiles
Carbenes
o Carbon with two bonds and one lone pair
o Neutral
o Rare
o sp2-hybridized
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o empty p-orbital
o can be electrophile or nucleophile
Radicals, carbocations, and carbanions are stabilized by resonance
o One away from a double bond is called “allylic”
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Being directly on the double bond is particularly unstable and this
position is called “vinyl”
o One away from an aromatic ring is called “benzylic”
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Being directly on the aromatic ring is particularly unstable and this
position is called “aryl”
The rest of chapter 4 that you should know from Gen Chem
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Equilibrium Constant
o Keq=
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o If Keq> 1, then more products than reactants
o If 0<Keq<1, then more reactants than products
o He may ask you to propose a Keq for a reaction based on some information.
 If he tells you a reaction is highly exergonic, then your Keq should be
really large.
 Think 103 or higher
 If he tells you a reaction is highly endergonic, then your Keq should be
really small.
 Think 10-3 or smaller.
 If he wants one of these extremes and you give a value like 1.5 or .7, he
won’t give full credit.
∆G=∆H-T∆S
o If ∆G<0, then spontaneous
 This is called exergonic.
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o If ∆G=0, then at equilibrium
o If ∆G>0, then non-spontaneous
 This is called endergonic.
Bond-Dissociation Energies
o If ∆H<0, then the reaction is exothermic.
o If ∆>0, then the reaction is endothermic.
o ∆H of a reaction can be estimated by
 ∆H=bonds broken-bonds formed
Kinetics
o For A + B → C + D
o rate=k[A]m[B]n
o The overall reaction order is m + n
o A is of the mth order and B is of the nth order
Activation Energy
o The difference in energy from the reactants to the highest-energy transition
state
Transition States
o Chemical species which exist fleetingly
o Structure or species when bonds are breaking and/or forming all at once
o Occur at the peaks of a reaction coordinate diagram
Intermediates
o Chemical species which occur at the troughs of a reaction coordinate diagram
o Exist long enough to be isolated
 This means that he will not use the word fleetingly when describing
them.
 Reaction coordinate diagrams
o The number of humps is the number of steps in a reaction.
o The valleys are intermediates.
o At the top of each hump is a transition state.
o The highest hump is the rate-determining step.